Extract date rendering into per-feature Date components

Elternbeirat.Web/Features/Posts/PostDateFormat.cs

@@ -0,0 +1,55 @@
+using System.Globalization;
+using Elternbeirat.Web.Shared;
+
+namespace Elternbeirat.Web.Features.Posts;
+
+/// <summary>
+///     Formats a post's calendar day for display and for a machine-readable
+///     <c>datetime</c> attribute.
+/// </summary>
+/// <remarks>
+///     Posts carry a <see cref="DateOnly"/> (a calendar day, no time of day). Their
+///     <c>&lt;time&gt;</c> element needs the day twice: once in German for the
+///     visitor (<see cref="Display"/>) and once as ISO 8601 for the <c>datetime</c>
+///     attribute (<see cref="Iso"/>). Keeping both here means the two never drift
+///     apart and the markup no longer repeats the same <c>ToString</c> pair.
+///     <para>
+///         This is the post counterpart to
+///         <see cref="Events.EventDateFormat"/>, which formats a
+///         <see cref="System.DateTime"/> range instead.
+///     </para>
+/// </remarks>
+public static class PostDateFormat
+{
+    /// <summary>
+    ///     Display format for a calendar day, e.g. <c>8. Oktober 2026</c>. The
+    ///     weekday is omitted, unlike the event format.
+    /// </summary>
+    private const string LongDate = "d. MMMM yyyy";
+
+    /// <summary>
+    ///     Formats a calendar day in German for a visitor to read.
+    /// </summary>
+    /// <param name="value">
+    ///     The calendar day to format.
+    /// </param>
+    /// <returns>
+    ///     The day as e.g. <c>8. Oktober 2026</c>, using German month names.
+    /// </returns>
+    public static string Display(DateOnly value)
+        => value.ToString(LongDate, Cultures.German);
+
+    /// <summary>
+    ///     Formats a calendar day as ISO 8601 (<c>yyyy-MM-dd</c>) for a
+    ///     machine-readable <c>datetime</c> attribute.
+    /// </summary>
+    /// <param name="value">
+    ///     The calendar day to format.
+    /// </param>
+    /// <returns>
+    ///     The day as e.g. <c>2026-10-08</c>, culture-invariant so the value is
+    ///     stable regardless of the request's culture.
+    /// </returns>
+    public static string Iso(DateOnly value)
+        => value.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);
+}